1. Field of the Invention
The present invention relates to a recording method that jets a viscous substance in droplets onto a recording medium according to an electric signal for a recording operation, and an apparatus for carrying out the same.
2. Description of the Related Art
An ink-jet recording system jets ink droplets through minute nozzles directly onto a recording medium, such as a paper sheet to form an image. Ink-jet printers of this ink-jet recording system are used widely as office and domestic printers since they meet the requirements of low cost, high quality, small size and color rendition.
Such printers are classified roughly by system into those of two systems, i.e., printers of a continuous system that jet ink droplets continuously and make only useful ones of the ink droplets reach a recording medium according to an image signal, and printers of a drop-on-demand system that jet only useful ink droplets discontinuously according to an image signal. The latter printers are classified further into those of a piezoelectric system that jet ink droplets by deforming ink passages by the vibration of piezoelectric devices, those of a thermal system that jet ink droplets by pressure produced by bubbles formed in the ink by heat generated by heating elements disposed in ink passages, and those of an electrostatic system that jet ink droplets by electrostatic attraction. The printer of an electrostatic system is being watched with interest recently because of its recording head of simple construction, and its capability of facilitating the fabrication of a multinozzle recording head and of forming gradated images by pulse duration modulation.
A common disadvantage of the ink-jet system is the spread of ink dots printed (recorded) on a recording medium. The ink dots printed on a recording medium dry as the ink penetrates into the recording medium and the volatile constituents of the ink evaporate. Therefore, the ink dots printed on a noncoated paper sheet or a fabric are caused to spread along fibers by capillarity to form a blurred image. Ink dots printed on a film or a metal surface that does not absorb the ink at all take a considerably long time to dry or to be fixed because the ink is unable to penetrate into the film or the metal surface. When recording a full-color image, ink dots are printed before previously printed ink dots dry and hence the boundaries between color regions are blurred.
The foregoing problems can be solved by using a viscous ink having a high viscosity. However, it is usual for the conventional ink-jet printing technique to use inks having a viscosity on the order of several centipoises (cP=0.01 P) because the ink-jet printing technique is able to produce a low force for jetting the ink. The viscosity of the ink which can be used for ink-jet printing is 20 cP at the highest.
A generally known method of solving problems relating to spreading without using viscous inks forms an ink-adsorbing layer on a surface of a recording medium. However, such a recording medium is expensive and the ink-absorbing layer spoils the unique texture of paper sheets and films. Therefore such a recording medium is used only in the field where the cost and the texture of the recording medium are insignificant. Accordingly, it has been believed that most of the conventional image printing methods of printing images on paper sheets and films cannot be replaced with the ink-jet printing method.
A solid ink-jet printing method proposed to overcome the foregoing disadvantages melts an ink which is a solid at an ordinary temperature or reduces the viscosity of such an ink and jets the ink. The ink employed in the solid ink-jet printing method contains a wax as a principal component. The ink is melted by heating the same at about 100xc2x0 C., the molten ink is supplied to and jetted through a nozzle. An ink dot formed by jetting the molten ink through the nozzle onto a recording medium solidifies rapidly and hence the ink dot does not spread on the recording medium. However, melting the ink takes time and hence a long warm-up time is required. The ink must be heated all the time after warm-up to keep the ink in a molten state. Ink dots printed on a recording medium rise on the recording medium, the rising ink dots invoke a sensation of heavy quality and often form lustrous images, which is a problem concerning design.
Another method to overcome the foregoing disadvantages applies a liquid composition that acts on an ink and exercises an effect of preventing the spread of ink dots to a recording medium prior to the injection of the ink onto the recording medium. Preferably, the liquid composition gelates when the same comes into contact with a color ink to solidify the color ink. This method, however, needs to apply an excessive second liquid to the recording medium to prevent the spread of the ink. Consequently, ink dots rise, the paper sheet becomes wavy and a long drying time is necessary.
The size of ink dots formed on a recording medium by the conventional ink-jet printing technique is several times as large as the diameter of the orifices of nozzles.
The process of formation of ink dots in such a size will be explained with reference to FIG. 24. As shown in FIG. 24, a viscous substance 202 contained in a nozzle 201 is extruded through the orifice of the nozzle 201 by electrostatic attraction or electromechanical force. An extruded portion 203 of the viscous substance 202 lengthens downward in a thread, the thread severs at its upper end, i.e., a portion contiguous with the orifice of the nozzle 201, after lengthening to a certain length, the thread is changed into a spherical droplet 204 by surface tension, and the spherical droplet adheres to a recording medium 205 in a dot. Thus, the size of the dot adhering to the recording medium 205 is five to six times as large as the diameter of the orifice of the nozzle 201. The diameter of the orifice of the nozzle 201 must be reduced to form a smaller dot. Such a nozzle is liable to be clogged with an ink containing large particles, and a portion of the nozzle defining the orifice is abraded by the particles contained in the ink thereby shortening the life of the recording apparatus.
The conventional ink-jet printing technique has difficulty in timing an operation for jetting a viscous substance and controlling the amount of the viscous substance to be jetted. More concretely, the electrostatic system, for instance, jets ink droplets at a predetermined frequency, charges the ink droplets, and make the charged ink droplets fly through a passage between deflecting electrodes to form ink dots. Jetting of the ink is controlled by the on-off control of voltage application across the deflecting electrodes. Therefore, ink droplets which need not form ink dots must be recovered by an ink droplet recovering device, which makes the printing apparatus complicated and large.
The present invention has been made in view of those problems and it is therefore a first object of the present invention to provide a method and an apparatus capable of jetting a viscous substance having a viscosity of 300 cP or above for patterning (recording), which could not have been achieved by the conventional ink-jet printing technique.
A second object of the present invention is to provide a method and an apparatus capable of forming images on recording mediums including plain paper sheets, films and metal surfaces without permitting spreading and without requiring the formation of a color material recipient layer, such as an ink adsorbing layer or a layer of a liquid composition capable of preventing spreading.
A third object of the present invention is to provide a method and an apparatus capable of suppressing the enlargement of dots formed on a recording medium.
A fourth object of the present invention is to provide a method and an apparatus capable of simply and reliably timing an operation for jetting a viscous substance and controlling the discharge amount of the viscous substance.
According to a first aspect of the present invention, a recording method using a viscous substance comprises the steps of: extruding a drop of the viscous substance from an open forward end of a nozzle filled up with the viscous substance; applying an electric signal corresponding to an image signal to a recording electrode disposed near the open end of the nozzle to vibrate the extruded drop of the viscous substance; and attaching a portion of the extruded drop of the viscous substance to a recording medium disposed opposite to the nozzle by separating the portion of the extruded drop of the viscous substance from the extruded drop of the viscous substance.
Preferably, the recording method according to the first aspect of the present invention further comprises the step of applying a pressure to the viscous substance filling up the nozzle in synchronism with the application of the electric signal to the recording electrode. Preferably, the recording method further comprises the step of heating the extruded drop of the viscous substance extruded from the open forward end of the nozzle. Preferably, the recording method further comprises the step of curing the viscous substance adhering to the recording medium with ultraviolet rays.
Preferably, the electric signal used in the step of applying the electric signal is a voltage pulse signal having a plurality of voltage pulses. Preferably, the timing of an operation for discharging droplets of the viscous substance is controlled by controlling the pulse width of the voltage pulses of the voltage pulse signal. Preferably, the discharge amount of the viscous substance is controlled by controlling the amplitude of the voltage pulses of the voltage pulse signal.
According to a second aspect of the present invention, a recording apparatus using a viscous substance comprises: a nozzle filled up with the viscous substance; a recording electrode disposed near an open forward end of the nozzle; and an electric signal applying unit for applying an electric signal corresponding to an image signal to the recording electrode to vibrate a drop of the viscous substance extruded from the open forward end of the nozzle.
It is preferable, in the recording apparatus according to the second aspect of the present invention, that the electric signal is a voltage pulse signal having a plurality of voltage pulses and the timing of an operation for discharging the viscous substance is controlled by controlling the pulse width of the voltage pulses of the voltage pulse signal. Preferably, the electric signal is a voltage pulse signal having a plurality of voltage pulses, and the electric signal applying unit controls the discharge amount of the viscous substance by controlling the amplitude of the voltage pulse signal.
As apparent from the foregoing description, according to the present invention, a pattern of a viscous substance having a viscosity of 300 cP or above can be recorded, which could not have been achieved by the conventional ink-jet printing technique.
According to the present invention, sharp and unblurred images can be formed on recording mediums including plain paper sheets, fabrics, films and metal surfaces because the viscous substance is used as an ink.
According to the present invention, the tip of a drop of the viscous substance extruded from the open forward end of the nozzle and having the shape of a circular cone is brought into contact with a recording medium and hence a small dot of the viscous substance can be formed on the recording medium.
According to the present invention, the timing of the operation for discharging the viscous substance and the discharge amount of the viscous substance can simply and accurately be controlled simply by varying the pulse width, the waveform or the amplitude of the voltage pulses of the voltage pulse signal and, therefore, a pattern of the viscous substance can very easily be formed.
The present invention can be applied not only to processes for forming images but also to processes for forming irregularities, such as a process for forming ribs for a plasma display and a process for forming the gap medium of a liquid crystal display. The present invention is particularly effectively applicable to the formation of the gap medium of a liquid crystal display.
In the liquid crystal display, minute glass beads are dispersed in a space between a pair of glass substrates to secure a gap for a liquid crystal. Those glass beads are impediments to displaying images by the liquid crystal. An improvement forms projections of a photoresist selectively in a non-displaying region (shaded region) of a color filter by a resist pattern forming process, which makes processes complicated and reduces the yield of the processes. The present invention forms projections selectively by small droplets directly on the color filter, which simplifies the manufacturing process and the gap medium can be formed at a small material loss.